Retinoid receptors

Retinoic acid (RA) is a hydrophobic ligand.  Its receptor exists in the nucleus.
Functional receptor is a heterodimer consisting of two closely similar proteins
named retinoic acid receptor (RAR) and retinoid X receptor (RXR).  The genes for
these two proteins belong to the nuclear receptor superfamily.  In vertebrates,
the nuclear receptors have been well characterized.  The heterodimer binds specific
DNA sequences and activates transcription of its target genes depending on ligand.
   We isolated RAR and RXR cDNA clones from Polyandrocarpa misakiensis.  This
is the first report of invertebrate RAR.  We demonstrated that the Polyandrocarpa
RAR/RXR can bind specific DNA sequences (Figure) and activate a reporter gene
transcription depending on RA.

Figure: Electrophoretic mobility shift assay demonstrating that the ascidian
RAR/RXR complex binds to a specific DNA sequence.  DR5 oligonucleotide probe
contains a strong binding site for vertebrate RAR/RXR heterodimers.  AP2 probe
contains a specific binding site for a transcription factor AP2.  AP2 protein binds
to AP2 probe but not to DR5 probe.  The mixture of ascidian RAR and RXR can
bind to DR5 probe but not to AP2 probe.  When either RAR or RXR only were
added to the binding reaction, electrophoretic mobility of DR5 DNA did not shift,
suggesting that RAR and RXR bind to the DR5 sequence as a heterodimer.


Hisata, K., Fujiwara, S., Tsuchida, Y., Ohashi, M. & Kawamura, K. (1998)
   Expression and function of a retinoic acid receptor in budding ascidians.
   Develop. Genes Evol. 208: 537-546.
Kamimura, M., Fujiwara, S., Kawamura, K. & Yubisui, T. (2000) Functional
   retinoid receptors in budding ascidians. Dev. Growth Differ. 42: 1-8.
Fujiwara, S., Kamimura, M., Ohashi, M. & Kawamura, K. (2001) Molecular bases
   of bud development in ascidians. In The Biology of Ascidians. (ed. Sawada, H.,
   Lambert,C. & Yokosawa,H.) Elsevier Science Publisher, pp.300-304.